The present invention relates generally to the manufacture of ceramic matrix composite articles, and more particularly to an improved method for manufacturing a composite article comprising a glass or glass-ceramic matrix within which is provided a reinforcing phase consisting of carbon or other inorganic fibers.
Composites comprising a matrix of glass and a reinforcing phase composed of refractory inorganic whiskers or fibers are well known. U.S. Pat. No. 4,314,852, for example, discloses glass products wherein reinforcing silicon carbide fibers are included to increase the fracture toughness and/or flexural strength of the product. U.S. Pat. No. 4,511,663 discloses fiber reinforced products wherein graphite fibers coated with a metal constitute the fiber reinforcement, while U.S. Pat. No. 4,626,515 describes a family of composites wherein reinforcement is by a combination of silicon carbide fibers and silicon carbide whiskers.
In conventional practice, composites such as disclosed in the prior art are fabricated by combining a yarn or tow of the fiber selected for reinforcement with a ceramic powder consisting of the matrix material to be used. To combine the yarn with the powdered matrix, a slurry of the matrix powder is prepared and the yarn is then coated with this slurry by passing a continuous strand of the yarn through the slurry. The slurry immersion thoroughly coats and impregnates the yarn with the powdered matrix.
The coated yarn thus produced is then collected, on a rotating drum or other support, to form a continuous layer or sheet, termed a prepreg. After drying, this sheet is typically cut into green prepreg mats comprising the fibers, typically in more or less uniaxial orientation, and intimately coated with the matrix powder mixture.
To provide a consolidated ceramic matrix composite product from prepreg mats produced as described, the mats or sections cut therefrom are stacked to a desired thickness and are then consolidated by the application of heat and pressure. The product of this hot pressing step is typically a dense composite article substantially free of voids and defects and exhibiting a strength and toughness substantially in excess of the matrix material alone.
As will be appreciated, the process of hot pressing adds considerable time and expense to the composite manufacturing process. Hot pressing cannot be carried out as a continuous process, and in addition is largely limited in its application to the consolidation of preforms of flat or relatively simple configuration.
Another problem related to the conventional processing of ceramic matrix composites as above described stems from the relatively low density of the composite prepreg material. Thus substantial debulking of the prepreg must normally take place before useful consolidation of the glass and fiber components can take place. For example, prepregs produced by slurry treatment of fiber tows as above described will typically have a density which is only about 30% of the final target density of the material. Thus a substantial volume reduction in the prepreg is required in the hot pressing step before the final density of the finished product can be reached.
A number of suggestions for debulking composite prepregs prior to final consolidation have been made. These include preliminary cold pressing of the prepregs, pultrusion of the impregnated yarn at ambient or elevated temperatures, and similar compacting procedures. However, these approaches again add processing steps and expense, and in many cases fail to achieve a useful level of debulking or provide prepreg sheet or yarn which is inflexible and relatively difficult to handle.
Accordingly, it is a principal object of the present invention to provide a process for ceramic matrix composite matrix manufacture which more economically provides high density ceramic matrix composite preforms or products.
It is a further object of the invention to provide a process for the fabrication of curved cylindrical or other hollow ceramic composite bodies wherein the necessity for hot pressing to reach a useful level of composite density is avoided.
Other objects and advantages of the invention will become apparent from the following description thereof.